Method of Forming Self-Aligned Buried Contact

The present invention relates to a method of forming a buried contact, especially to a method of forming a self-aligned buried contact of a metal-oxide semiconductor field effect transistor (MOSFET) in a high density integrated circuit.

Generally speaking, in the process of fabricating MOSFET in high density integrated circuit, polysilicon is often used as conducting wires between MOSFET elements to reduce the use of metal wires, therefore it is necessary to use buried contact structure. Conventional buried contact process is illustrated in FIGS. 1a to 1e which show the cross-sectional view of each step.

As shown in FIG. 1a, a field oxide 10 and a gate oxide 11 are grown on a silicon substrate 1. After a first polysilicon layer 12 and a first photoresist 13 are deposited, a buried contact region 14 is formed by defining the first photoresist 13 and etching.

As shown in FIG. 1b, the first photoresist 13 is removed to finish buried contact etch, then a second polysilicon layer 15 is deposited. Impurities could be implanted and driven in to improve the conductivity of the second polysilicon layer 15.

FIG. 1c shows a tungsten silicate (WSi.sub.x) layer 16 is deposited.

FIG. 1d shows that a second photoresist 17 is deposited and defined.

As shown in FIG. 1e, a buried contact region 18 is formed by anisotropic etching, and then the second photoresist 17 is removed.

Another conventional buried contact process is shown in FIGS. 2a to 2d. It is different from the above-mentioned process because tungsten silicate layer is not used. For convenience, corresponding elements use same numerals in all embodiments.

As shown in FIG. 2a, a field oxide 10 and a gate oxide 11 are grown on a silicon substrate 1. After a first polysilicon layer 12 and a first photoresist 13 are deposited, a buried contact region 14 is formed by defining the first photoresist 13 and etching.

As shown in FIG. 2b, the first photoresist 13 is removed to finish buried contact etch, then a second polysilicon layer 15 is deposited. Impurities could be implanted and driven in to improve the conductivity of the second polysilicon layer 15.

FIG. 2c shows that a second photoresist 17 is deposited and defined.

As shown in FIG. 2d, a buried contact region 18 is formed by anisotropic etching, and then the second photoresist 17 is removed.

The most often encountered problem of the conventional buried contact process is that although conducting wires are formed between MOSFET elements by polysilicon, if the misalignment of definition of the second photoresist is too large, the polysilicon layer 15 will not contact with the buried contact region 18 which leads to disconnection of wiring and therefore induces failure of the MOSFET elements.

It is therefore an object for the present invention to provide a method of forming a buried contact, using self-alignment technique to prevent failure of semiconductor elements due to disconnection of wiring which is caused by misalignment.

The object of the present invention is fulfilled by providing a method for forming a self-aligned buried contact on a semiconductor substrate having a field oxide and a gate oxide grown thereon and a first polysilicon layer deposited thereon and a buried contact region formed thereon by etching, comprising the following steps of: depositing a second polysilicon layer thereon while forming a recess on the buried contact region; forming a sidewall spacer within the recess; depositing a tungsten silicate layer thereon; depositing and defining a second photoresist thereon; anisotropically etching the tungsten silicate layer and the second polysilicon layer; and removing the second photoresist.

The present invention will be more fully understood from the detailed description given hereinafter with reference to and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein:

FIGS. 1a to 1e show cross-sectional views of a conventional buried contact process;

FIGS. 2a to 2d show cross-sectional views of another conventional buried contact process;

FIGS. 3a to 3f show cross-sectional views of an embodiment of the self-aligned buried contact process according to the present invention; and

FIGS. 4a to 4e show cross-sectional views of another embodiment of the self-aligned buried contact process according to the present invention.

A preferred embodiment of the present invention is illustrated in FIGS. 3a to 3f and described hereinafter.

Step 1

As shown in FIG. 3a, a field oxide 10 and a gate oxide 11 are grown on a silicon substrate 1. After a first polysilicon layer 12 and a first photoresist 13 are deposited, a buried contact region 14 is formed by defining the first photoresist 13 and etching.

Step 2

As shown in FIG. 3b, the first photoresist 13 is removed to finish buried contact etch, then a second polysilicon layer 15 is deposited. Impurities could be implanted and driven in to improve the conductivity of the first polysilicon layer 12.

Step 3

As shown in FIG. 3c, a sidewall spacer 19 is formed in a recess of the second polysilicon layer 15 above the buried contact region 14. This could be done by depositing a silicon dioxide layer or a silicon nitride layer with a thickness of 1 K.ANG. to 1.5 K.ANG., and etching back using anisotropic etching technique.

Step 4

FIG. 3d shows a tungsten silicate layer 16 having a thickness of 1 K.ANG. to 1.5 K.ANG. is deposited.

Step 5

FIG. 3e shows that a second photoresist 17 is deposited and defined.

Step 6

As shown in FIG. 3f, a buried contact region 18 is formed by anisotropic etching, and then the second photoresist 17 is removed.

Another preferred embodiment of the self-aligned buried contact process according to the present invention is illustrated in FIGS. 4a to 4e and described hereinafter.

Step 1

As shown in FIG. 4a, a field oxide 10 and a gate oxide 11 are grown on a silicon substrate 1. After a first polysilicon layer 12 and a first photoresist 13 are deposited, a buried contact region 14 is formed by defining the first photoresist 13 and etching.

Step 2

As shown in FIG. 4b, the first photoresist 13 is removed to finish buried contact etch, then a second polysilicon layer 15 is deposited. Impurities could be implanted and driven in to improve the conductivity of the first polysilicon layer 12.

Step 3

As shown in FIG. 4c, a sidewall spacer 19 is formed in a recess of the second polysilicon layer 15 above the buried contact region 14. This could be done by depositing a silicon dioxide layer or a silicon nitride layer with a thickness of 1 K.ANG. to 1.5 K.ANG., and etching back using anisotropic etching technique.

Step 4

FIG. 4d shows that a second photoresist 17 is deposited and defined.

Step 5

As shown in FIG. 4e, a buried contact region 18 is formed by anisotropic etching, and then the second photoresist 17 is removed.

As stated above, since a sidewall spacer is formed above the buried contact region in the buried contact process according to the present invention, the tolerance of misalignment is greatly increased because the polysilicon layer will contact with the buried contact region if the polysilicon layer could contact the sidewall spacer, so that self-alignment is achieved and the problem of disconnection in the conventional buried contact process is reduced.

While the invention has been described by way of examples and in terms of several preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.